The invention relates to a device for dispensing jets of cryogenic fluid as well as an installation and a working method using the said jets, in particular jets of liquid nitrogen, under high pressure, in particular an installation and method for surface treatment, scouring, cleaning or descaling of materials, coated or not, such as metals, concrete, wood, polymers, ceramics and plastics materials or any other type of material.
Currently the surface treatment of materials, coated or not, in particular scouring, descaling or the like, is done essentially by blast cleaning, by spraying water at ultrahigh pressure (UHP), by sanding machine, by scaling hammer, by bush hammer or by chemical method.
However, when there must not be any water there, for example in a nuclear environment, or chemical product, for example because of drastic environmental constraints, only so-called “dry” working methods may be used.
However, in some cases, these “dry” methods are difficult to implement, are very laborious or difficult to use or give rise to additional pollution, for example because of the addition of shot or sand to be reprocessed thereafter.
One alternative to these technologies is based on the use of cryogenic jets under very high pressure as proposed by the documents U.S. Pat. No. 7,310,955 and U.S. Pat. No. 7,316,363. In this case, one or more jets of liquid nitrogen are used at a pressure of 300 to 4000 bar and at a cryogenic temperature between for example −100° and −200° C., typically approximately −140° C. and −160° C., which are dispensed by one or more nozzles driven or not in a rotary movement.
Normally, at a pressure of around 3500 bar, and for a nozzle diameter of around 300 μm, a jet of cryogenic fluid, in particular a jet of liquid nitrogen, typically has a maximum coherence length of around 15 to 18 cm. Coherence length means the length of the jet of cryogenic fluid over which the jet remains sufficiently concentrated so as to be visible after its escape through the nozzle.
However, the effective length of a jet of cryogenic fluid is also a very important characteristic since it corresponds to the maximum distance from the ejection nozzle beyond which the jet is no longer sufficiently concentrated to maintain its effectiveness in surface treatment, scouring, cleaning or descaling of the material being treated. The effective length is consequently less than or equal to the jet coherence length, which is the visible jet length.
In other words, the greater the effective length of the jet, the more effective the working method is for equal distance between the nozzle and the substrate being treated, and the more the method gains in efficiency, the said efficiency corresponding for example, in the case of a concrete descaling method, to the volume of concrete descaled per unit of time.
Thus, for a jet of cryogenic fluid to be effective and able to implement the required working method, it is necessary for the surface of the material treated to be situated, with respect to the outlet of the jet dispensing nozzle, at a distance less than or equal to the effective length and therefore less than the coherence length of the said jet. This effective jet length is in some cases, that is to say depending on the working method in question, small, that is to say around a few centimeters and typically between 5 and 15 mm for a jet of cryogenic fluid at a pressure of around 3500 bar dispensed by a nozzle with a diameter of around 300 μm. The tolerance in positioning of the jet dispensing nozzle with respect to the surface of the material is then problematic.
This is because it is technically difficult to maintain a strictly fixed distance between the jet dispensing nozzle and the surface of the material treated, whether the method be used manually or automatically, when the material has on its surface a defect in flatness or surface condition or roughnesses, that is to say a succession of hollows and projections, as is the case with concrete for example.
Thus, if the unevenness or the depth of the hollows has an excessively great amplitude, the areas of material treated situated at these defects or hollows are situated at a greater distance from the nozzle outlet, at which the jet has lost all or some of its effectiveness, which leads to a working method that is less effective in these areas. The working method is then less reliable, which is critical for some applications, such as the cleaning of contaminated parts in a nuclear environment, for which the least residue of pollution is not acceptable.
Moreover, an effective length of the jet that is insufficient makes a method of working implemented on a part in which features such as conduits or tracks are produced very difficult or even impossible. The problem posed is then even more critical since the bottom of the conduit or track being treated may be situated beyond the effective length of the jet, and because of this out of the range thereof, thus making the working method of low effectiveness, or even ineffective, in this area.
Moreover, the fact that the conventional jets of cryogenic fluid have a coherence length and therefore an effective length in general less than 20 cm poses a problem for the treatment, in particular cleaning, of heat exchangers used in for example installations of the power station, hydrocarbon desulphurisation factory, air or water treatment factory type, where the heat exchangers may have diameters greater than 40 cm. In this case, the part treated, that is to say the exchanger, consists itself of parts some of which are situated at more than 20 cm from the circumference of the said exchanger, and which it is necessary to be able to clean, which is not possible with the cryogenic fluid jets of the prior art.
The problem addressed is consequently proposing a method of working by cryogenic fluid jets that is improved, that is to say for which the drawbacks related not only to the limited coherence length but also to the limited effective length of the jets no longer exist or are greatly reduced, and thus making the working method using the said jets more reliable and more effective.